1. Field of the Invention
This invention pertains to an apparatus for testing body fluid constituents. More particularly, this invention pertains to a needle assembly for a sampler for use in collecting body fluids such as interstitial fluid.
2. Description of the Prior Art
In the prior art, there are numerous examples of apparatus for testing and determining the level of constituents in human blood. A great deal of attention has been directed to the development of techniques and apparatus for measuring blood glucose.
As noted in commonly assigned and co-pending U.S. patent application Ser. Nos. 08/321,305 and 08/136,304 (corresponding to PCT International Publication No. WO95/10223 published Apr. 20, 1995 on International Application No. PCT/US94/11580 and incorporated herein by reference), the determination of the level of a constituent of blood can be achieved by measuring the level of that constituent in other body fluids such as interstitial fluid. The aforementioned patent applications and international publication disclose a method and apparatus for a minimally invasive technique for collecting a sample of interstitial fluid through use of an extremely small needle which penetrates into the dermal layer of the skin in order to collect a low blood or blood-free sample of interstitial fluid. The collected interstitial fluid can then be analyzed for a determination of the level of constituents within the fluid. For example, the collected interstitial fluid can be analyzed for an amount of glucose with the determined amount being representative of the amount of glucose contained within the patient's blood.
The aforementioned applications and international publication disclose a membrane onto which collected fluid is deposited by the needle. The needle and membrane are mutually arranged such that a plane of an open end of the needle is perpendicular to a plane of the membrane. Stated another way, the longitudinal axis of the needle is parallel to and spaced from the surface of the membrane. As a result of this arrangement, fluid flowing from the needle to the membrane must make a 90.degree. turn in its flow path to be deposited on the membrane. Such a tortuous flow path is an impediment to maximizing a volumetric flow from the needle to the membrane.
Also, although the intent is to draw a blood-free sample, small numbers of blood cells may, from time-to-time, flow through the needle with collected interstitial fluid. Such blood cells may interfere with the reliability of testing of a collected sample if such cells are deposited on the membrane in an area through which a testing light passes.